1. Field of the Invention
The present invention relates to a thermal transfer sheet and more particularly to a thermal transfer sheet which can produce a high-quality image even on a paper having a rough surface.
2. Background Art
A thermal transfer sheet comprising a substrate film having a thermal transfer ink layer on one side thereof has hitherto been used for carrying out output printing for a computer or a word processor.
This conventional thermal transfer sheet is prepared by coating a mixture of wax with a colorant, such as a pigment or a dye, onto a substrate film of a paper, such as a 10 to 20 .mu.m-thick capacitor paper or paraffin paper, or a plastic film, such as a 3 to 20 .mu.m-thick polyester or cellophane film, to form a thermal transfer ink layer on the substrate film.
One drawback of the conventional thermal transfer sheet is that, when a rough paper having a poor surface smoothness is used as an object for printing, a transferred ink layer as a print has void-like defects or dropouts, making it impossible to provide a good print.
Various proposals have been made with a view to eliminating the above drawback. For example, Japanese Patent Laid-Open No. 59747/1994 proposes a thermal transfer sheet comprising; a substrate film; and a hot-melt layer (a release layer) composed mainly of wax, a color layer (a thermal transfer ink layer) unmeltable at the time of thermal transfer, and an adhesive layer composed mainly of wax provided in that order on one side of the substrate film.
This thermal transfer sheet has an advantage that, since the release layer is composed mainly of wax, it can be easily melted and brought to a low-viscosity liquid, resulting in improved releasability of the thermal transfer ink layer to provide a high-quality print. This advantage is obtained when the thermal transfer sheet is used in combination with a high-speed printer, for example, a high-speed printer having a serial thermal head used in word processors. The use of low- to medium-speed printers, that is, cold release type printers (printers wherein the melting of the release layer and the transfer of the ink layer do not occur simultaneously and there is a timelag therebetween), however, does not offer this advantage.
The reason for this is that, during the timelag, the melted wax is cooled and becomes viscous or is solidified to create a heat-sealing property, reducing the difference in transferability between a print area and a non-print area. That is, the function of the release layer is lowered, and this tendency becomes significant when the object is a rough paper, posing a problem that no high-precision and high-resolution image can be provided.
Japanese Patent Laid-open No. 183882/1988 proposes a thermal transfer sheet having a layer construction of substrate film/release layer/thermosoftening layer with any one of the layers being colored. This thermal transfer sheet is characterized in that the thermosoftening layer is formed of an emulsion of an ionomer resin (a resin crosslinked with a metal ion). The thermal transfer sheet is described to enable a high-quality print to be provided on a rough paper. In fact, however, the print quality varies depending upon energy applied at the time of thermal transfer, and it is difficult for the energy applied in the low- to medium-speed printer to provide a print having a satisfactorily high quality on a rough paper.
Accordingly, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer sheet which can form an image having a high quality on a rough paper.